Polar and Ferrel cells
The Coriolis effect
Any object moving toward or away from the equator and not firmly attached to the surface does not travel in a straight line. As the diagram illustrates. It is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Moving air and water tend to follow a clockwise path in the Northern Hemisphere and a counterclockwise path in the Southern Hemisphere.
The French physicist Gaspard Gustave de Coriolis (1792-1843) discovered the reason for this in 1835, and it is called the Coriolis effect. It happens because the Earth is a rotating sphere, and as an object moves above the surface, the Earth below is also moving. The effect used to be called the Coriolis "force," and it is still abbreviated as CorF, but it is not a force. It simply results from the fact that we observe motion in relation to fixed points on the surface.
The Earth makes one complete turn on its axis every 24 hours. This means every point on the surface is constantly moving and returns to its original position (relative to the Sun) every 24 hours, but because the Earth is a sphere, different points on the surface travel different distances to do so. If you find it difficult to imagine that New York and Bogotá—or any other two places in different latitudes—are moving through space at different speeds, consider what would happen if this were not so: the world would tear itself apart.
Consider two points on the surface, one at the equator and the other at 40° N, which is the approximate latitude of New York and Madrid. The equator, latitude 0°, is about 24,881 miles (40,033
km) long. That is how far a point on the equator must travel in 24 hours, which means it moves at about 1,037 MPH (1,668 km/h). At 40° N, the circumference parallel to the equator is about 19,057 miles (30,663 km). The point there has less distance to travel and so it moves at about 794 MPH (1,277 km/h).
Suppose you planned to fly an aircraft to New York from the point on the equator due south of New York (and could ignore the winds). If you headed due north, you would not reach New York. At the equator you are already traveling eastward at 1,037 MPH (1,668 km/h). As you fly north, the surface beneath you is also traveling east, but at a slower speed the farther you travel. If the journey from 0° to 40° N took you six hours, in that time you would also move about 6,000 miles (9,654 km) to the east, relative to the position of the surface beneath you, but the surface itself would also move, at New York by about 4,700 miles (7,562 km). Consequently, you would end not at New York, but (6,000 - 4,700 =) 1,300 miles (2,092 km) to the east of New York, way out over the Atlantic, somewhere due south of Greenland.
The size of the Coriolis effect is directly proportional to the speed at which the body moves and the sine of its latitude. The effect on a body moving at 100 MPH (160 km/h) is 10 times greater than that on one moving at 10 MPH (16 km/h). Sin 0° = 0 (the equator) and sin 90° = 1 (the poles), so the Corio-lis effect is greatest at the poles and zero at the equator.
The polar easterlies and mid-latitude westerlies meet along the boundary between polar and tropical air. This is known as the polar front. At the top of the polar front, close to the tropopause, the temperature difference on either side is very large. It produces the polar front jet stream, which is a strong, high-level wind blowing from west to east in both hemispheres.
Air rises along the polar front. Some of the air flows back toward the pole, where it cools and subsides once more. This produces a second set of cells, called the polar cells.
Some of the air flows toward the equator. Over the Tropics it meets the high-level air of the Hadley cells and subsides with it. This forms a third set of cells. Their existence was discovered by the American meteorologist William Ferrel (1817-91) and they are known as Ferrel cells.
There is another region of light surface winds, known as the horse latitudes, where the subsiding air reaches the surface and divides. Historically, ships often carried cargoes of horses. If the ships were becalmed, supplies of drinking water sometimes ran short. Horses often died of thirst when this happened, and their bodies were thrown overboard, hence the name.
The Hadley cells and polar cells are direct cells, driven by convection and the subsidence of cold, dense air. The Ferrel cells are indirect cells, driven by the direct cells to the north and south of them. Together, the Hadley, Ferrel, and polar cells comprise the three-cell model shown in the diagram.
Between them, these cells transport warm air away from the equator and cool air toward the equator. Without them, tropical temperatures would be much higher than they are and polar temperatures much lower.
Three-cell model. The tropical (Hadley) and polar cells are directly driven by convection. The middle-latitude (Ferrel) cell is indirect, because it is driven by the polar and tropical cells.
Continue reading here: Transport Of Heat By The Oceans
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